The roughly 200 cubic miles of ash would form into an "umbrella cloud" and spread across the U.S. in a radial pattern. Ashfall would measure 3 feet deep in much of the northern Rocky Mountain region to under an inch in places as far away as New York.

Umbrella clouds are not formed by smaller eruptions, as the ash plumes are not shot nearly as high, meaning they don't get caught in the jet stream and transported to far-away places. An eruption from Yellowstone's supervolcano, despite sitting deep underground, would hurl ash miles into the air, and spread out in a relatively even bull's eye pattern.

"In essence, the eruption makes its own winds that can overcome the prevailing westerlies, which normally dominate weather patterns in the United States," USGS scientist Larry Mastin told Raw Story.

"This helps explain the distribution from large Yellowstone eruptions of the past, where considerable amounts of ash reached the West Coast."

The Yellowstone supervolcano, also known as a caldera, has produced eruptions of this magnitude at least three times, 2.1 million, 1.3 million, and 640,00 years ago.

The modern historical record has no events comparable to Yellowstone's ancient eruptions, but the scientists note that Indonesia's Mount Tambora eruption of 1815 "cooled the planet enough to produce the famed 'year without a summer' in 1816, during which snow fell in June in eastern North America and crop failures led to the worst famine of the 19th century."

The researchers note that they are 99.9-percent sure that a Yellowstone eruption will not occur this century, stating clearly that, "Geological activity at Yellowstone provides no signs that a supereruption will occur in the near future."

When the supervolcano does decide to wake up, we'll have ample warning.

"Yellowstone hasn't erupted for 70,000 years, so it’s going to take some impressive earthquakes and ground uplift to get things started. Besides intense earthquake swarms we expect rapid and notable uplift around the caldera," the researchers wrote in a Q&A with the USGS.